1. Field of the Invention
The present invention relates to a wireless LAN, and more particularly, to a Dynamic Host Configuration Protocol environment in a wireless LAN.
2. Description of Related Art
As wireless LAN technology has been developed, several types of wireless LAN standards have been introduced. The most widely used of these wireless standards is the IEEE (institute of electrical and electronics engineers) 802.11 wireless LAN standard. Further, a DHCP (Dynamic Host Configuration Protocol) defined in RFC (Request for Comments) 1531 has been generally used as a standard for assigning an IP address to a mobile node in a wireless LAN environment based on the IEEE 802.11.
FIG. 1 illustrates the configuration of a conventional communication system. Referring to FIG. 1, the conventional communication system includes a mobile node 11, an access point A 21, an access point B 22, an access point C 23, an access router A 31, and an access router B 32.
The mobile node 11 freely moves from network to network and is equipped with a wireless LAN function and a DHCP client function. The access point A 21, the access point B 22, and the access point C 23 are equipped with wireless LAN and wired LAN functions, and communicate wirelessly with the mobile node 11 existing in the network managed by the access points A 21, B 22, and C 23. The access points A 21 and B 22 communicate with the access router A 31 managing the access points A 21 and B 22, and the access point C 23 communicates with the access router B 32 managing the access point C 33 via wire. Accordingly, the access points A 21, B 22 and C 23 connect the mobile node 11 to the access routers A 31 and B 32. In particular, the access point C 26 is equipped with a DHCP server function. The access router A 33 and the access router B 34 are equipped with a wire LAN function and a routing function, and communicate with an access point in a subnet that the access router A 33 or B 34 independently manages by wire.
It is assumed that the mobile node 11 passes sequentially through networks managed by the access point A 21, the access point B 22, and the access point C 23, as illustrated in FIG. 1.
Since the access router managing the access point A 21 and the access router managing the access point B 22 are the same router, i.e. the access router A 31 when the mobile node 11 moves from a network the access point A 21 manages to a network the access point B 22 manages, a subnet is not changed.
Since the access router managing the access point B 22 and the access router managing the access point C 23 are the different routers, i.e. the access router A 31, the access router B 32 when the mobile node 11 moves from the network the access point B 22 manages to the network the access point C 23 manages, a subnet is changed, and a new IP address to be used in the new subnet is required. The process of acquiring an IP address in a new subnet will now be described.
When a subnet where the mobile node 11 exists is changed, that is, when the mobile node 11 moves from a network the access point B 22 manages to a network the access point C 23 manages, the mobile node 11 associates with the access point C 23 by transmitting an association request frame 41 to and receiving an association response frame 42 from the access point C 23 based on the IEEE 802.11 wireless LAN standard.
When the mobile node 11 has become associated with the access point C 23, the mobile node 11 searches for a DHCP server by broadcasting a DHCPDISCOVER message based on the DHCP defined in RFC 1531.
When access points corresponding to the DHCP server receive the broadcasted DHCPDISCOVER message from the mobile node 11, the access points transmit DHCPOFFER messages to the mobile node 11. The mobile node 11 selects the access point C 23 among the access points providing the DHCPOFFER messages, and requests an IP address by broadcasting a DHCPREQUEST message indicating that the access point C 23 was selected.
When the access point C 23 receives the broadcasted DHCPREQUEST message from the mobile node 11, the access point C 23 transmits a DHCPACK message containing an IP address to the mobile node 11. The mobile node 11 receives the DHCPACK message from the access point C 23, and uses the IP address contained in the DHCPACK message as its own IP address.
As described above, a total of four messages must be transmitted and received to acquire an IP address based on the conventional DHCP, and thus, it takes much time for the mobile node to be assigned a new IP address. In addition, since the access point corresponding to a DHCP server can respond and assign the IP address to the mobile node, only after a mobile node corresponding to a DHCP client sends a request to an access point, the access point cannot assign IP addresses independently.